1. Field of the Invention
The present invention relates to an imaging lens and an imaging apparatus. In particular, the present invention relates to a wide-angle imaging lens that is appropriate for use in an in-vehicle camera, a monitor camera, or the like that uses an imaging device, such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor). Further, the present invention relates to an imaging apparatus including the imaging lens.
2. Description of the Related Art
In recent years, the size of an imaging device such as a CCD and a CMOS became very small, and the resolution of the imaging device became very high. Therefore, the size and the weight of the main body of imaging equipment and an imaging lens mounted on the imaging equipment also need to be reduced. Meanwhile, imaging lenses used in the in-vehicle camera (on-vehicle camera), the monitor camera (or a surveillance camera) and the like need to have excellent weather-resistance characteristics. Further, the imaging lenses need to have wide angles of view and excellent optical performance so that an excellent view is ensured for a wide range.
Further, since it is desirable to reduce the cost for producing the imaging lens in the aforementioned fields, an optical system composed of a small number of lenses is desirable. Conventionally, for example, Japanese Unexamined Patent Publication No. 2002-244031 (Patent Document 1), U.S. Pat. No. 7,280,289 (Patent Document 2), U.S. Pat. No. 7,375,906 (Patent Document 3), Japanese Unexamined Patent Publication No. 2005-227426 (Patent Document 4), and U.S. Pat. No. 7,518,809 (Patent Document 5) disclose imaging lenses, each composed of four lenses, in the aforementioned fields.
Meanwhile, in the fields of the in-vehicle camera, the monitor camera, and the like, a demand for wider angle lenses increased in recent years. For example, lenses having full angles of view exceeding 180° became desirable. Further, as the size of the imaging device became smaller, and the resolution of the imaging device became higher in recent years, an imaging lens that has high resolution and high optical performance that enables obtainment of excellent images for a wide image formation range became desirable. However, in the conventional lens system, it was difficult to realize an imaging lens that has a wider angle of view and high optical performance, and which can satisfy the demand in recent years, while the lens system is constructed at low cost and in small size.
Patent Document 1 describes, as a wide angle lens in Example 3 thereof, a lens system composed of four lenses of first through fourth lenses, which are sequentially arranged from the object side of the wide angle lens. In the wide angle lens, an aperture stop is arranged between the third lens and the fourth lens. Patent Document 1 is silent about the F-number of the lens system and the angle of view of the lens system. However, since the refractive index of the first lens is approximately 1.52, and the negative power of the first lens and the negative power of the second lens are relatively weak, it is improbable that the lens system can cope with a specification in which a full angle of view exceeds 180°.
The full angles of views of the lenses disclosed in Patent Documents 2 and 3 are in the range of approximately 140° to 165° and in the range of approximately 152° to 164°, respectively. Therefore, they do not satisfy a need for a wider angle lens that has a full angle of view exceeding 180° in recent years. Further, in the lens disclosed in Patent Document 4, the F-number is in the range of 2.5 to 2.8, and the full angle of view is greater than or equal to 180°. However, when a projection method, in which an ideal image height is represented by 2×f×tan (φ/2) using focal length f of the entire lens system and half angle φ of view, is adopted, distortion (distortion aberrations) sharply increases on the minus side when the half angle of view exceeds 80°. Therefore, the lens disclosed in Patent Document 4 has a disadvantage that an image in the most peripheral area thereof becomes small. Further, Patent Document 5 discloses an example in which a full angle of view is close to 190°. In Patent Document 5, both of the distortion and the lateral chromatic aberration of the lens are corrected in an excellent manner. However, astigmatism remains. Further, when the lens is used in combination with a higher-resolution imaging device, a deeper depth of field is required in some cases.